The Biogas Production

The Biogas Production by AgribioEnergia

Thanks to the will of some farmers to support their farms during a difficult period for the agriculture, the Agribioenergia Società Cooperativa Agricola was founded in 2005.

The aim was to set up a plant for the anaerobic digestion of various organic “matrices” such as corn, sorghum, cattle slurry, and by-products of agroindustry processes, in order to obtain biogas and as a consequence electricity and renewable heat.

The system consists of:

  • Three storage silos. Once a year, during the month of August, the agricultural product to be used for biomasses, is harvested (corn and sorghum). This product is deposited in silos in well-pressed piles with the purpose of properly conserving them for a long time. The biomass is then daily used to supply the biogas plant.
  • A filling hopper. It is a large container that gathers the daily quantity of solid biomasses to be used to supply the plant. This hopper is filled once a day with matrices and quantities depending on the supply recipe.
  • A pre-filling tank. It is a tank that gathers the matrices in liquid form, mainly agro industrial by-products, which are measured out in the plant every hour in function of the supply recipe and the availability of by-products.
  • Two fermenters, the primary and the secondary one. These two tanks accommodate all the biomasses, both the liquid and the solid ones. In these tanks, the biomasses are kept at a constant temperature of about 40 degrees centigrade, by means of a heating system that recovers the heat produced by the generator. The biomasses are always mixed through mixers. The biomass fermentation occurs here, caused by the various colonies of methanogenic bacteria, which – digesting the biomasses – produce biogas, that is rich in methane.
  • A post-fermenter. It is a tank where the “digestate” from the primary and secondary fermenters is collected. It is also maintained at a constant temperature and the “digestate” is always mixed, in order to “extract” biogas from the biomasses. This tank functions as gasometer: it looks like a big ball and contains the biogas produced by the plant, creating a “lung” effect before the gas is sent to the cogeneration process.
  • A solid-liquid separator. It is a machine for squeezing and separating the solid from the liquid part of the “digestate”, when the biomass is completely digested. The solid part is re-used to cover the silage piles within the silos, while the liquid part is sent to the storage tank.
  • The storage tank. It is a big tank that can contain clarified “digestate” during the winter months, when it is not possible to use it as fertilizer on the fields.
  • A treatment and cleaning plant for the biogas. This plant has the purpose of purifying the biogas to be used by the cogenerator.
  • The cogenerator. It is a big endothermic engine connected to a current generator, which uses the biogas as a fuel. Consequently, it produces electricity that is sent into a power line, and thermal energy, obtained by the cooling circuit of the engine and by the recovery of the heat contained in the exhaust gas.

The Digestate

An important aspect of this process is precisely the “digestate”. The biomasses, during the brew cycle, which can last from 120 to 180 days, undergo some transformations, including methane extraction and the mineralization of nutrients such as nitrogen, phosphorus and potassium thus making them directly assimilated by plants and becoming a completely organic fertilizer.

In this process the matrices are fully stabilized, eliminating any unpleasant smells; the use of this “digestate” as fertilizer, has allowed a significant reduction of the use of synthetic fertilizers on our fields.

The system can indeed activate a chain, which turns agricultural production of raw materials ans secondary raw materials

into electricity and heat without air emissions (not exceeding the emissions of a large vehicle fuelled by methane) and with no waste production.
All this happens under the direct control of the agricultural enterprise, with the main purpose of remunerating the contributing shareholder farmers.

Over the years, the cooperative has progressively refined the choice of the crops appropriate for the biomasses. In particular, the cooperative still tries to identify and select the most productive plants varieties, that require less water and technical means thus having the maximum yield.

The Biogas Future

Another objective of the cooperative is to seek and identify agricultural by-products to supply the plant, replacing progressively the cultured biomasses. As of today, more than the half of the introduced biomasses are made up by by-products.
Thanks to these elements of study and research, it has become possible, between 2012 and 2016, to reduce by 30% the hectares of dedicated crops.

The Board of Directors is still investing in research and initiatives for the development of the cooperative. These are aimed at using the thermal energy that is currently not exploited and enhanced by the plant.

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